Prevent the event loop from pausing when entering modal loop (eventlo…

…op-2.0) (rust-windowing#839)

* Prevent the event loop from pausing after entering modal loop

After clicking the window title bar or border (for a drag or resize),
the event loop pauses until the mouse is moved. This change relays
the WM_NCLBUTTONDOWN message to the dummy window where it queues
a redraw and consumes the message. This effectively jumpstarts
the modal loop and it continues to fire draw requests.

* Handle WM_NCLBUTTONDOWN in public_window_callback instead of relaying.

Relaying the WM_NCLBUTTONDOWN message to the modal window turned out
to be unnecessary.

src/platform_impl/windows/event_loop.rs

@@ -799,6 +799,20 @@ unsafe extern "system" fn public_window_callback<T>(
             commctrl::DefSubclassProc(window, msg, wparam, lparam)
         },
 
+        winuser::WM_NCLBUTTONDOWN => {
+            // jumpstart the modal loop
+            winuser::RedrawWindow(
+                window,
+                ptr::null(),
+                ptr::null_mut(),
+                winuser::RDW_INTERNALPAINT
+            );
+            if wparam == winuser::HTCAPTION as _ {
+                winuser::PostMessageW(window, winuser::WM_MOUSEMOVE, 0, 0);
+            }
+            commctrl::DefSubclassProc(window, msg, wparam, lparam)
+        },
+
         winuser::WM_CLOSE => {
             use event::WindowEvent::CloseRequested;
             subclass_input.send_event(Event::WindowEvent {